Radar apparatuses detect target objects (e.g., aircrafts and ships) generally by transmitting pulses of electromagnetic waves from antennas and receiving reflection waves from the target objects from the antennas. There are cases where reflective bodies which generate sea and rain clutter and the like (unnecessary reflection wave) exist around the target objects to be detected. JP1998-197627A and “Revised Radar Technique” (supervised by Takashi Yoshida, Institute of Electronics, Information and Communication Engineers, 1996, p. 217) disclose Doppler processing for suppressing clutter.
The reflection waves from the target objects received by the antennas of the radar apparatuses are normally sampled at an equal interval for every transmission azimuth of the electromagnetic wave. In the Doppler processing, data obtained by converting the reflection waves (echo signals) from the target objects is used to perform Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) in order to obtain a Doppler spectrum. The digitalized echo signal is decomposed into a plurality of sub-bands by a filter bank (multiple Doppler filters) relating to a Doppler shifted frequency. Data outputted from each sub-band is Constant False Alarm Rate (CFAR) processed and then is combined with each other.
Meanwhile, there is a case where a radar apparatus equipped in a ship (referred to as “the ship”) receives an influence due to an electromagnetic wave outputted from another radar apparatus which is equipped in a ship existing around the ship (the ship equipped with the other radar apparatus is referred to as “the other ship”), and mutual interference occurs between the radar apparatuses.
As one of means for suppressing the influence of mutual interference between the radar apparatuses, JP3561497B discloses a staggered trigger scheme. The staggered trigger scheme is a scheme in which processing of changing a transmission interval of repeatedly transmitting the pulse signal (stagger processing) is applied. By performing the stagger processing, it can be avoided that a reception timing of an echo signal from a target object caused by a pulse signal transmitted by a radar apparatus matches with a reception timing of a pulse signal transmitted by another radar apparatus between sweeps. If the reception timings do not match between sweeps, the echo signal from the target object can be discriminated from the transmission signal caused by the radar apparatus of the other ship based on the reception signals in the plurality of sweeps.
However, in the case where the stagger processing is performed in the radar apparatus to which the Doppler processing is applied, if a relative speed of the other ship with respect to the ship increases, a signal-to-clutter ratio (S/C) and a signal-to-noise ratio (S/N) may not be able to improve as sufficiently as expected in the Doppler processing, due to the influence of the stagger processing.